17. He sends down water (rain) from the sky, and the valleys flow according to their measure, but the flood bears away the foam that mounts up to the surface, and (also) from that (ore) which they heat in the fire in order to make ornaments or utensils, rises a foam like unto it, thus does Allah (by parables) show forth truth and falsehood . Then, as for the foam it passes away as scum upon the banks, while that which is for the good of mankind remains in the earth. Thus Allah sets forth parables.

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Tafsir Ibn Kathir Volume 5, Page 259

This honorable Ayah contains two parables which affirm that truth remains and increases, while falsehood diminishes and perishes. Allah said,

﴿أَنزَلَ مِنَ الْسَّمَآءِ مَآءً﴾

“He sends down water from the sky”,

﴿فَسَالَتْ أَوْدِيَةٌ بِقَدَرِهَا﴾

“and the valleys flow according to their measure”,

each valley taking its share according to its capacity, for some valleys are wider and can retain more water than others which are small and thus retain smaller measures of water. This Ayah indicates that hearts differ, for some of them can retain substantial knowledge while others cannot entertain knowledge, but rather are bothered by knowledge,

﴿فَاحْتَمَلَ السَّيْلُ زَبَدًا رَّابِيًا﴾

“but the flood bears away the foam that mounts up to the surface”

of the water that ran down the valleys; this is the first parable. Allah said next,

“and (also) from that (ore) which they heat in the fire in order to make ornaments or utensils…”

This is the second parable, whereas gold and silver ore is heated with fire to make adornments with it, and iron and copper ore are heated to make pots and the like with it. Foam also rises to the surface of these ores, just as in the case with water,

﴿كَذَلِكَ يَضْرِبُ اللَّهُ الْحَقَّ وَالْبَـطِلَ﴾

“thus does Allah (by parables) show forth truth and falsehood.”

when they both exist, falsehood does not remain, just as foam does not remain with the water or the gold and silver ores which are heated in fire. Rather, foam dissipates and vanishes,

﴿فَأَمَّا الزَّبَدُ فَيَذْهَبُ جُفَآءً﴾

“Then, as for the foam it passes away as scum upon the banks,”

for it carries no benefit and dissipates and scatters on the banks of the valley. The foam also sticks to trees or is dissipated by wind, just as the case with the scum that rises on the surface of gold, silver, iron and copper ores; it all goes away and never returns. However, water, gold and silver remain and are used to man’s benefit. This is why Allah said next,